1. Field of the Invention
The present invention relates to a photomask and a projection exposure apparatus used for the lithographic processes in the fabrication of semiconductor devices and liquid crystal devices.
2. Related Background Art
For a projection exposure apparatus (a stepper, for example), it has hitherto been practiced that the image of circuit patterns formed on a photomask (a reticle) is projected to be imaged on a photosensitive substrate (a semiconductor wafer or a glass plate with photoresist coated on the surface thereof) through a projection optical system. The reticle used for an apparatus of the kind is such that a shielding member (chrome or other metallic film) is provided to adhere to a substrate (quartz or other glass plate) which is transparent with respect to the illuminating light for exposure, and the circuit patterns formed by the light transmitting portion (the bare surface portion of the substrate) and the shielding portion are transferred onto the photosensitive substrate.
FIG. 20 is a view schematically showing the structure of a conventional projection exposure apparatus. FIG. 21 is a view showing the specific structure of a reticle shown in FIG. 20. In FIG. 20, the illuminating light for exposure (i line, excimer laser, or the like) IL emitted from an illumination system (not shown) is reflected by a dichroic mirror DCM to illuminate a reticle R stacked on a reticle stage RS with substantially even illuminance. The illuminating light IL transmitted through the pattern area PA of the reticle R enters the projection optical system PL which is telecentric on both sides. A projection optical system PL projects the image of circuit patterns to be imaged on the wafer.
As shown in FIG. 21, there are formed in the reticle R, four alignment marks RX.sub.1, RX.sub.2, RY.sub.1, and RY.sub.2 in the shielding zone (chrome layer) LSB having a specific width surrounding the pattern area PA. The four alignment marks are all transparent windows or linear type or cross type marks formed in a transparent window. Further, on the peripheral part of the reticle R, two reticle marks (cross type marks in FIG. 21) RM.sub.1 and RM.sub.2 are oppositely arranged. In this respect, any one of the marks on the reticle R is formed in the same process for the circuit patterns to be formed.
Two sets of reticle alignment systems RA.sub.1 and RA.sub.2 shown in FIG. 20 are as disclosed in U.S. Pat. No. 4,710,029, for example, provided to detect the reticle marks RM.sub.1 and RM.sub.2 by the application of a synchronous wave detection method. The reticle alignment systems RA.sub.1 and RA.sub.2 are used for positioning the reticle R with respect to the optical axis AX of the projection optical system PL by detecting the reticle marks RM.sub.1 and RM.sub.2 through mirrors MR.sub.1 and MR.sub.2.
Also, four sets of alignment sensors AS.sub.X1, AS.sub.X2, AS.sub.Y1, and AS.sub.Y2 all irradiate illuminating light of a wavelength range different from that of the exposure light IL onto the alignment marks on the reticle R through the dichroic mirror DCM and further irradiate the alignment marks formed on the peripheral part of the shot area on the wafer through the reticle R (transparent window) and the projection optical system PL. Moreover, the sensors detect the rays of light emitted from both marks photoelectrically thereby to detect the amount of the relative misregisteration between the reticle and the shot area. The sensors are arranged for each of the four alignment marks (transparent windows) RX.sub.1, RX.sub.2, RY.sub.1, and RY.sub.2. Applications have been filed as Ser. No. 687,944 (Apr. 19, 1991) and Ser. No. 888,828 (May 27, 1992) for the alignment sensors such as described above.
As shown in FIG. 22A, when a spot light SP and an alignment mark AL.sub.8 on the reticle are scanned relatively in the direction Y, it is possible to obtain from a photoelectric detector a photoelectric signal as shown in FIG. 22B if the light transmitted through the reticle is received by the photoelectric detector. In accordance therewith, the mark position can be detected. Therefore, from each of the positions of plural alignment marks, the shifting amounts are calculated both in the directions X and Y as well as in the rotational direction of the reticle. Thus, a desirably accurate positioning of the reticle is possible by minutely adjusting the reticle stage so that the aforesaid shifting amounts will become zero.
In recent years, there has been proposed the use of a phase shift reticle provided with a phase shifter (dielectric thin film or the like) which can shift the phase of the light transmitted through a specific portion in the transmittable portions of a circuit pattern by .pi. (rad) with respect to the light transmitted through the other transmittable portion thereof, instead of the reticle formed only by a light shielding member (metallic film). (Hereinafter, this latter reticle is referred to as an ordinary reticle). If the phase shift reticle is used, it becomes possible to perform a pattern exposure with a high resolution and great focal depth as compared with the ordinary reticle.
As regards the phase shift reticle, there have been proposed various methods, but the typical ones are of a spatial frequency modulation type, shifter light shielding type, and half tone type. For the spatial frequency modulation type phase shift reticle, there is a disclosure in Japanese Patent Publication 62-50811, for example, wherein a phase shifter is arranged to adhere to either one of the transmittable portions having therebetween light shielding patterns which are arranged at constant pitches. Also, for the shifter light shielding type phase shift reticle, there is a disclosure in Japanese Patent Laid-Open Application 4-165352, for example, wherein the structure is arranged only with the phase shift pattern having its width narrower than the resolution limit of a projection optical system to be employed. Further, for the half tone type phase shift reticle, there is an application filed as Ser. No. 780,249 (Oct. 22, 1991), for example, wherein the structure is arranged only with a semitransparent pattern for which the phase of transmitted light is shifted only by .pi. (rad) and its transmittivity is defined to be approximately 15%.
In the phase shift reticles, particularly in the shifter light shielding type and half tone type, if the alignment marks are formed in the same process as the formation of the circuit patterns, a problem is encountered in that it becomes impossible to detect alignment marks in the conventional mark detection system. In other words, an alignment mark APL shown in FIG. 23A is formed only with the phase shifter in the shifter light shielding type while in the half tone type, it is formed only with the semitransparent member. Therefore, when the spot light SP and the alignment mark APL are relatively scanned, a photo-electric signal such as shown in FIG. 23B is obtained from the photoelectric detector. This means that the amount of the transmitted light is lowered only at the edge which is extended in the direction X of the alignment mark APL, and that even if the spot light SP is superposed on the mark APL, the amount of the transmitted light obtainable is still equal to the amount obtainable from the portion other than the mark APL.
As a result, when a phase shift reticle is employed, it is impossible to detect the mark position on the reticle accurately by use of the conventional projection exposure apparatus. There is thus a problem that the accuracy of the reticle alignment, the alignment between the reticle and wafer, the base line measurement or the like is lowered. In order to prevent this, it is necessary to form reticle marks with chrome and the like in a process other than the circuit pattern formation process or to provide a measurement instrument dedicated for the purpose. In either case, there is encountered a problem that the manufacturing cost will be increased.